This invention relates to an improved porous alumina filter body formed from an extruded monolith substrate. The body is formed by sintering mixtures containing aluminum and alumina powders in an oxidizing atmosphere (e.g., air). During sintering of the body, oxidation of the metal occurs with a concomitant expansion that counteracts the shrinkage caused by sintering, giving an overall volume change that is negligibly small or zero. The resulting body is highly permeable to gases and liquids, and may be used for filtration purposes or as a support for a semi-permeable membrane.
Ceramic Honeycomb Monoliths. Extruded ceramic honeycomb monoliths were initially developed as catalyst supports for automotive catalytic converters, environmental catalyst supports for fixed site installations, and diesel particulate filters. These monoliths have a multiplicity of passageways that extend from one end face to an opposing end face. The cell structure is formed by the extrusion process, with a cell density as low as low as 9 cells per square inch (cpsi) to as high as 1400 cpsi. For monoliths with circular cross sections, diameters can be as large as 12 inches, or greater. The length of such monoliths in extrusion can be over 6 feet, and is limited by such factors as the available facilities for uniform drying and sintering. Numerous patents exist for such monoliths produced from cordierite (e.g., Lachman and Lewis in U.S. Pat. No. 3,885,997, and Frost and Holleran in U.S. Pat. No. 3,899,326) and silicon carbide (e.g., Stobbe in U.S. Pat. No. 5,195,319 and U.S. Pat. No. 5,497,620). In general, such monoliths, especially those with larger diameters and longer lengths, are difficult to produce from most ceramics. Cordierite has been produced relatively readily because it has a low coefficient of thermal expansion (CTE) of about 2xc3x9710xe2x88x927/xc2x0 C. to 1xc3x9710xe2x88x926/xc2x0 C. This low CTE minimizes thermal and mechanical stresses during the sintering and cool-down process, allowing sintering of such monoliths and avoiding fracture during sintering. Silicon carbide monoliths with a higher CTE of about 3.5-4xc3x9710xe2x88x926/xc2x0 C. have superior thermal and mechanical properties that permit their sintering and cooling without fracturing. First, the relatively high thermal conductivity of silicon carbide (e.g.,  greater than 5W/m-K) relieves thermal gradients within the monolith. Second, the relatively high mechanical strength allows greater stress tolerance during sintering and cool-down. Third, and relevant to the subject of this invention, the volume change during sintering is very small, typically about 1-2% shrinkage. This xe2x80x9cnear net shapexe2x80x9d property is important for sintering ceramic bodies in a way that minimizes internal stresses during the sintering cycle, thereby reducing the risk of mechanical failure.
In summary, present methods for manufacturing honeycomb-structured monoliths involve extrusion of suitably plastic batch materials through a die, followed by drying and sintering at an appropriate temperature to produce the final monolith. The choice of materials currently available for monolith fabrication is restricted to those that have a very low CTE or exhibit negligible ( less than 2-5% linear) shrinkage during sintering in order to prevent deformation and/or cracking of the monolith channels during sintering and subsequent cooling. At present, honeycomb-structured monoliths are only commercially available in relatively low CTE materials, such as cordierite, mullite and silicon carbide. Although cordierite and mullite are relatively inexpensive materials, their chemical durabilities are inferior to those of silica-free oxide ceramics. The chemical durability of silicon carbide is significantly greater, but the relatively high fabrication cost associated with sintering at elevated temperatures ( greater than 2000xc2x0 C.) in an inert atmosphere make the use of silicon carbide an expensive proposition for many engineering applications. Also, when used for certain applications, including those involving membrane coatings, silicon carbide monoliths may suffer from a chemical durability limitation. Specifically, the surface of silicon carbide is readily oxidized to silica. The bond between an overlying membrane coating and this silica interface may be subject to chemical attack, especially by alkaline solutions.
Porous Ceramic Monoliths as Membrane Supports. Porous ceramic monoliths are widely used as supports for filter bodies and ceramic membrane filter devices, and the patent art contains descriptions of monoliths produced from many different materials. Perhaps the earliest disclosure was in the French Patent Publication 2,061,933, filed Oct. 3, 1969 by the Commissariat a L""Energie Atomique, which describes a multichannel xcex1-alumina monolith as a support for an xcex1-alumina ultrafiltration membrane. In 1978 Hoover and Roberts (U.S. Pat. No. 4,069,157) described the use of cordierite honeycomb monoliths as supports for dynamically formed membranes. In 1984, Gillot, et al., presented a paper xe2x80x9cNew Ceramic Filter Media for Cross-Flow Microfiltration and Ultrafiltrationxe2x80x9d (Filtra 1984 Conference, Oct. 2-4, 1984) that described the use of sintered xcex1-alumina membranes deposited on sintered xcex1-alumina monoliths supports, closely related to the CEA French patent mentioned above. Abe, et al. (U.S. Pat. No. 4,894,160) disclosed the use of clay-bonded xcex1-alumina as a honeycomb monolith support. In 1993 Faber and Hudgins described the use of titania as a monolith membrane support (U.S. Pat. No. 5,223,318). In 1995 Castillon and Laveniere (U.S. Pat. No. 5,415,775) disclosed the use of a mixture of titania/xcex1-alumina monoliths as membrane supports. Grangeon and Lescoche describe metal oxide monolith supports containing titania in combination with other metal oxides, especially alumina (U.S. Pat. No. 5,607,586 and U.S. Pat. No. 5,824,220).
In general, porous xcex1-alumina, configured in tubular and monolith structures, is the most common material used as a support for ceramic membranes. Such porous xcex1-alumina materials are most commonly produced by sintering a monodisperse alumina at temperatures of 1600xc2x0 C. to 1800xc2x0 C. The use of clay, or other metal oxides, or fine xcex1-alumina reactive binders can reduce the sintering temperature needed.
Large diameter honeycomb monoliths have been used for membrane supports for crossflow membrane devices. For example, the patents of Hoover and Roberts (U.S. Pat. No. 4,069,157), Hoover and Iler (U.S. Pat. No. 4,060,488), Goldsmith (U.S. Pat. No. 4,781,831, U.S. Pat. No. 5,009,781, and U.S. Pat. No. 5, 108,601), Faber and Frost (U.S. Pat. No. 5,641,332), Yorita, et al., (U.S. Pat. No. 5,855,781), and Rajnik, et al. U.S. Pat. No. 6,077,436) disclose such devices.
Similar large diameter monoliths have been used as dead end filters, especially for diesel exhaust gas filtration. Early diesel filter devices are described by Outland (U.S. Pat. No. 4,276,071), Higuchi, et al. (U.S. Pat. No. 4,293,357, U.S. Pat. No. 4,340,403 and U.S. Pat. No. 4,364,760), Berg, et al., (U.S. Pat. No. 4,364,761), Pitcher (U.S. Pat. No. 4,329,162 and U.S. Pat. No. 4,417,908), and other extensive patent art.
Similar monolith structures have been used as membrane supports for dead end membrane filters in which the monolith passageways are coated with a membrane and the passageway ends are plugged, for example, in an xe2x80x9calternate, adjacent checkerboard patternxe2x80x9d typical of diesel exhaust filters, Goldsmith, et al. (U.S. Pat. No. 5,114,581). These filters can be used for the removal of particulates from a gas or a liquid.
The above large diameter monoliths used as membrane supports (or filter bodies) have all been conceptual designs or made from ceramic materials (cordierite, mullite or silicon carbide) that can be successfully extruded, dried and sintered in large diameter parts while maintaining mechanical integrity. The decisive disadvantage of ceramics and ceramic composites formed by such a process is the normally high linear shrinkage that occurs between the green body and the final product, typically in the range of 5% to 15%. This shrinkage is problematic when trying to maintain the shape and dimension of a part. Shrinkage during sintering and cool-down can lead to the formation of cracks and other defects, up to and including the fracture of large parts.
Reaction Bonded Alumina Materials. Claussen has disclosed reaction-bonded alumina (RBAO) materials, in which xcex1-alumina and related ceramic bodies are sintered using precursors that show xe2x80x9cnear net shapexe2x80x9d on sintering, i.e., negligible volume change occurs on sintering the green (unfired) body to the sintered state (Claussen, U.S. Pat. No. 5,607,630). The RBAO process includes the use of metal and ceramic powders in the batch formulations to form green bodies. During heating, the metal powder undergoes a volumetric expansion as a consequence of oxidation, and this volume increase offsets the normal shrinkage due to sintering of the ceramic powder constituents. The work of Claussen and those of several other groups active in the RBAO field focus on fabrication of near net shape bodies with low to negligible porosity. This low porosity and small pore size is achieved, in part, because the metal and ceramic powders used in the forming of the bodies are reduced to about 1 xcexcm by aggressive attrition milling. Relatively high metal powder contents are also used, and this leads to filling of the pore volume during the oxidative expansion of the metal.
Claussen, et al., have also disclosed the fabrication of analogous near net shape ceramic bodies from other ceramic materials (Claussen, et al., in U.S. Pat. No. 5,326,519, U.S. Pat. No. 5,843,859, U.S. Pat. No. 6,025,065 and U.S. Pat. No. 6,051,277)
Variations on the chemistry of the process have also been reported, including the incorporation of ZrO2 in the RBAO body (Wu, et al., J. Am. Ceram. Soc., 76 (1993) 970), oxidation of a metallic Baxe2x80x94Fe precursor to barium hexaferrite (Ward and Sandhage, J. Am. Ceram. Soc., 80 (1997) 1508), and the oxidation of aluminum with SiC to form mullite/alumina/SiC composites (Wu and. Claussen, J. Am. Ceram. Soc., 77 (1994) 2898). The results of Wu, et al., are also included, in part, in the Claussen patents cited above. In all cases, however, the intention has been to form a dense ceramic part with essentially no open porosity.
Reaction Bonded Alumina Monoliths as Membrane Supports. The reaction-bonding mechanism of Claussen has been extended to alumina ceramic membranes by Andriansens, et al. of the Belgian institute V.I.T.O (EP 0,766,995 A1), which discloses the possible use of RBAO membrane supports. The characteristics of these RBAO membrane supports are more fully disclosed in V.I.T.O published technical papers (Luyten, et al., xe2x80x9cShaping of a RBAO membrane supportxe2x80x9d, Key. Eng. Mat., 132-136, 1691-1694 (1997); Vercammen, et al., xe2x80x9cPorous Reaction Bonded Alumina: Machining and Joining Propertiesxe2x80x9d, Key Eng. Mat., 132-136, 1703-1706 (1997); Luyten, et al., xe2x80x9cShaping of Multilayer Ceramic Membranes by Dip Coatingxe2x80x9d, J. Eur. Cer. Soc., 17, 273-279 (1997); and, Vercauteren, et al., xe2x80x9cPorous Ceramic Membranes: Preparation, Transport Properties and Applicationsxe2x80x9d, J. of Porous Materials, 5 (1998) 241-258). These supports are characterized in that the process of Claussen has been followed, and the RBAO membrane supports have been processed as supports for gas separation membranes. Very fine powder precursors and/or extensive milling were employed to reduce precursor powder size. As such, the pressed supports have a very fine pore size ( less than 0.2 xcexcm diameter) and correspondingly would have much lower permeability that the larger-pored, monolith-based membrane supports described above.
Yet another report of a RBAO support was given in a poster paper at the 6th International Conference on Inorganic Membranes, Montpellier, France, Jun. 26-30, 2000 (Ding, et al., xe2x80x9cExtruded Porous Reaction Bonded Alumina Support with Boehmite as Aidxe2x80x9d, paper P123). The supports were extruded with either polyvinyl alcohol (PVA) or boehmite as a binder. Supports with pore diameter of 0.65-0.85 xcexcm diameter and 41-43% porosity were obtained.
Still another research group active in development of RBAO membrane supports is at the Materials and Energy Research Center, Tehran, Iran (Falamaki, et al., xe2x80x9cRBAO Membranes/Catalyst Supports with Enhanced Permeabilityxe2x80x9d, J. Eur. Cer. Soc., 21, 2267-2274 (2001). This group has reported processes to make RBAO membrane supports using fine xcex1-alumina powder (ca. 1 xcexcm particle size) and lightly milled aluminum (ca. 1 xcexcm particle size). The difference in the preparation method of this group is that the oxidation of the aluminum occurs as a liquid (molten) aluminum-gas reaction in lieu of the solid-gas reaction by Claussen, et al. The supports of this group also have pore diameters well below 1 xcexcm.
In all the above RBAO work, either in the preparation of near dense parts or porous membrane supports, there is no evidence that any prior work involved the extrusion of porous honeycomb monoliths as described above. Moreover, for the limited amount of work performed for membrane supports, all structures had pore diameters below 1 xcexcm diameter. As disclosed in the monolith art for membrane supports above, porous supports with larger pore size and greater permeability are required to be suitable. This is especially true for the supports disclosed by Goldsmith (patents cited above) for monoliths used for crossflow filters, membrane-coated filters, and dead-end particulate filters. For these, the relatively high level membrane or filter surface area per unit volume of monolith ( greater than 100 ft2/ft3) and the associated high level of filtrate or permeate flow requires a highly permeable support. The analysis supporting this conclusion is presented in U.S. Pat. No. 4,781,831. Further analysis of the permeability requirements for high surface area monoliths with large diameter are disclosed in Hoover and Roberts (cited above).
It is therefore an object of this invention to provide an improved porous xcex1-alumina substrate in the form of an extruded, multiple-passageway monolith.
It is a further object to provide such a substrate that exhibits small to negligible volume change on sintering of the extruded green monolith.
It is a further object of this invention to provide such a porous xcex1-alumina substrate in a large diameter monolith with a large amount of surface area of the monolith passageway walls relative to the volume of the monolith.
It is a further object of this invention to provide such a substrate which has a mean pore size and porosity required to effectively serve as a monolith-based porous support for a pressure driven membrane device.
This invention results from the realization that the fabrication of such an xcex1-alumina monolith substrate requires a composition of the green monolith that undergoes minimal volume change on sintering, and that this can be accomplished by extruding a monolith which contains at least aluminum metal and relatively coarse alumina powders mixed in a proportion such that the volume change of the green monolith on sintering is controllably small. This invention also results from the realization that certain minimal pore size and porosity properties of the monolith are required for monolith-based composite filtration devices. Finally, this invention also realizes that incorporating an alumina (preferably xcex1-alumina) of a powder size above a certain minimum is needed to realize the minimal pore size for the effective use of such monoliths.
This invention features a porous xcex1-alumina substrate consisting of a sintered monolith of porous material defining a plurality of passageways extending longitudinally from one end face of the monolith along the length of the monolith to an opposing end face. The monolith is extruded from a mixture containing at least an aluminum metal powder and an alumina powder and dried to form a green monolith, the mixture containing the aluminum and alumina powders in a proportion controlled to minimize the volume change of the sintered monolith from that of the green monolith. The surface area of the passageways of the substrate is at least 100 ft2/ft3, and the mean pore diameter of the porous material is greater than about 1 xcexcm.
In a preferred embodiment of the xcex1-alumina substrate, the alumina powder in the mixture includes an xcex1-alumina powder with a mean particle size in the range of about 5-200 xcexcm. In another embodiment of the substrate, the alumina powder in the mixture can further include a fine reactive xcex1-alumina powder. Preferably, the sintered xcex1-alumina monolith shows a volume change on firing of less than about 5% from that of the green monolith. Also, an xcex1-alumina precursor can be included in the mixture, and this precursor forms xcex1-alumina on sintering of the green monolith. Such a precursor can be a transition alumina, aluminum hydroxide, a hydrated alumina, a soluble aluminum compound, and mixtures thereof. In addition, another metal oxide can be admixed with the mixture used to extrude the substrate. This metal oxide can be selected from the group consisting of zirconia, titania, magnesia, and mixtures thereof. The mixture can also include the addition of organic additives to facilitate extrusion and to bind the green body.
The substrate preferably has a hydraulic diameter of at least two inches. Also, the mean pore diameter of the porous material of the substrate is preferably 3 to 50 xcexcm, and the porosity of the porous material is preferably about 20 and 60 volume %.
One embodiment of this invention is a composite filtration device in which the xcex1-alumina substrate has a filtration membrane applied to the passageway wall surfaces of the monolith. The membrane can be selected from the group of membranes suitable for microfiltration, ultrafiltration, nanofiltration, pervaporation and gas separations. This invention features a porous xcex1-alumina substrate consisting of a sintered monolith of porous material defining a plurality of passageways extending longitudinally from one end face of the monolith along the length of the monolith to an opposing end face. The monolith is extruded from a mixture containing at least an aluminum metal powder and an alumina powder and dried to form a green monolith, the mixture containing the aluminum and alumina powders in a proportion controlled to minimize the volume change of the sintered monolith from that of the green monolith. The surface area of the passageways of the substrate is at least 100 ft2/ft3, and the mean pore diameter of the porous material is greater than about 1 xcexcm.
Another embodiment of this invention features a porous xcex1-alumina substrate consisting of a sintered monolith of porous material defining a plurality of passageways extending longitudinally from one end face of the monolith along the length of the monolith to an opposing end face. The monolith is extruded from a mixture containing at least an aluminum metal powder and an alumina powder and dried to form a green monolith, the mixture containing the aluminum and alumina powders in a proportion controlled to minimize the volume change of the sintered monolith from that of the green monolith. The particle size of the alumina powder is sufficiently large so that the mean pore diameter of the porous material is greater than about 1 xcexcm, and the surface area of the passageways of the substrate is at least 100 ft2/ft3.
A preferred embodiment of this invention features a porous xcex1-alumina substrate in the form of a sintered monolith of porous material defining a plurality of passageways extending longitudinally from one end face of the monolith along the length of the monolith to an opposing end face. In this embodiment, the monolith is extruded from a mixture containing at least an aluminum metal powder and an xcex1-alumina powder and dried to form a green monolith, the mixture containing the aluminum and alumina powders in a proportion controlled to minimize the volume change of the sintered monolith from that of the green monolith. The xcex1-alumina powder has a mean particle size in the range of 5-200 xcexcm; the mean pore diameter of the porous material is in the range of 3 to 50 xcexcm; the porosity of the porous material is between 20% and 60% by volume; the monolith has a hydraulic diameter of at least two inches; and the surface area of the passageways is at least 100 ft2/ft3 of the monolith.
This invention also includes a method for making a sintered, porous xcex1-alumina substrate in which a mixture is first formed containing at least a predetermined amount of an aluminum metal powder and an alumina powder. The mixture is extruded to form a monolith containing a plurality of passageways extending from one end face of the monolith along the length of the monolith to an opposing end face and dried to form a green monolith. The green monolith is sintered at a temperature sufficient to oxidize the aluminum metal powder and to bond the monolith and then cooled to ambient temperature. In this embodiment the predetermined amounts of the aluminum metal powder and the alumina powder in the mixture are chosen in a proportion to minimize the volume change of the sintered and cooled monolith from that of the green monolith.